In U.S. Pat. No. 3,019,127 a glass fibrous mat made of fibers measuring from approximately 20-25 microns is sprayed with an adhesive, carbon particles of 12 to 50 mesh are sprinkled on the pad, the pad vibrated to distribute the particles and the adhesive cured to adhere the particles in the mat. The method results in a relatively low carbon particle loading, i.e., on the order of 4 percent of particulate material per unit volume of pad. In U.S. Pat. No. 4,227,904, carbon particles measuring 12/28 to 4/6 (Tyler Screen Series) are glued to the face of a perforated substrate to provide a layer of particles on the substrate. This results in a medium loaded product. A highly loaded, thin-bed filter has been made by D-Mark, Inc. of Mt. Clemens, Mich. and others wherein the space between two perforated sheets is filled with loose carbon particles. This results in a high capacity filter, but the particles tend to settle resulting in channeling and shedding of carbon dust. As used herein, the term thin-bed filter refers to a filter having a bed or substrate measuring in thickness anywhere up to two inches.
It has been a long-sought objective to provide an odor-removing thin-bed filter with reasonably high carbon loading, low carbon shedding and no induced channeling at a low unit cost. In some instances, as for the commercial/industrial filter market, a highly efficient, high-capacity filter is desired, while for the range hood and appliance markets a somewhat less efficient and lower capacity filter may suffice. In providing filters for these two markets, allowable pressure drops must be adhered to. For example, in the range hood market, this pressure drop should preferably not exceed 0.15 inches of water column, while in the commercial/industrial market, 0.3 inches is typically allowable.
In the manufacture of the filter according to U.S. Pat. No. 4,227,904, relatively large, odor-removing, pelletized particles are employed such as 6/8 mesh (Tyler Screen Series). This size pelletized material has become difficult to acquire and substitute materials have been difficult to reliably adhere to the filter substrates. As the filter made under U.S. Pat. No. 4,227,904 was particularly adapted for manufacture with the 6/8 pellet, the need has arisen to find other approaches to the manufacture of thin-bed type filters.
In addition, there has been a need to provide a combination odor and grease removing filter that will visually indicate when it has become grease laden and should be replaced.
In seeking to provide suitable filters for both the range hood/appliance and the commercial/industrial markets at the lowest cost, it has become desirable to provide a method of manufacture that utilizes readily available granular carbon and is sufficiently flexible so filters especially designed for these different markets may be produced without requiring separate dedicated production lines. This has lead to the desirability of a filter wherein the odor-removing media is so suspended in the air stream that greater or lesser quantities of the media may be provided (in accordance with the different market requirements) by simple changes in the method of filter manufacture.